As far as my understanding goes, you can use your XP license on only one computer. With the activation junk, they now have a way to keep people from using their license on more than one computer. With any of the other Windows (3.1,95,98,ME,2k), you can legally only install it on 1 desktop computer and 1 laptop computer. Now, we know how well that worked (haha) so that's why M$ is doing the activation.
Now, personally, I finally got around to installing XP RC1 about two weeks ago. I think I set a record for myself: shortest time I've ever had an OS installed. I just didn't like it, period. Yea, it has better hardware support, but any noble piece of hardware is going to have stable Win2k drivers as well. So, you know what I did, I pulled out my Ghost floppy and restored my bootdrive back to Win2k. They way things are going, I think more programs are going to become available for Linux in the next couple years. Before I shell out major $$ for another M$ OS, I'm going to try out the latest version of Linux.